The present invention relates to a monitoring optical system, which is applicable to a scanning optical device such as a laser printer, for detecting a drawing-start timing or a drawing-end timing per scan. Particularly, the present invention relates to the monitoring optical system for the scanning optical device in which a laser beam deflected by a deflector is separated from a laser beam incident thereon in an auxiliary scanning direction.
U.S. Pat. No. 5,621,562 discloses the scanning optical device that has such an arrangement of the deflected laser beam separated from the incident laser beam.
FIG. 4 is a perspective view of the scanning optical device disclosed in the U.S. patent. A laser beam emitted by a semiconductor laser 10 passes through a collimator lens 11 and a cylindrical lens 12 and is reflected by a flat mirror 13 to be incident on a polygonal mirror (deflector) 14. The deflected laser beam by a reflection surface 14a of the polygonal mirror 14 is reflected by a curved surface mirror 15 and forms a beam spot on a photoconductive drum 18 (shown by two-dot chain line) through a drawing anamorphic lens 16 and an optical path bending mirror 17.
The beam spot scans on the photoconductive drum 18 as the polygonal mirror 14 rotates.
In this specification, a direction equivalent to the scanning direction of the beam spot on the photoconductive drum 18 is referred to as a main scanning direction, a direction perpendicular to the main scanning direction is referred to as an auxiliary scanning direction.
The laser beam at the surface of the photoconductive drum 18 is the reference point for defining the direction of the optical power of the optical elements. That is, the power in the main scanning direction means the power contributing to converge or diverge the laser beam in the main scanning direction at the drum 18. The power in the auxiliary scanning direction means the power which contributes to converge or disperse the laser beam in the auxiliary scanning direction at the drum 18.
The laser beam deflected by the polygonal mirror 14 is separated from the laser beam incident thereon in the auxiliary scanning direction, i.e., a direction of a rotation axis 14b of the polygonal mirror 14. Further, the laser beam reflected by the curved surface mirror 15 is also separated from the laser beam incident thereon in the auxiliary scanning direction.
The cylindrical lens 12 has a positive refractive power only in the auxiliary scanning direction for forming a line-spread image near the polygonal mirror 14. The curved surface mirror 15 primarily has a positive power in the main scanning direction and the drawing anamorphic lens 16 primarily has a positive power in the auxiliary scanning direction.
A monitoring flat mirror 40 is located between the curved surface mirror 15 and the drawing anamorphic lens 16. The monitoring flat mirror 40 separates a monitor beam from the deflected laser beam at a separation point outside of the scanning range. When the laser beam reflected by the curved surface mirror 15 reaches the end of the scanning range, the laser beam is reflected by the monitoring flat mirror 40 and is converged onto a monitoring sensor 42 through a monitoring cylindrical lens 41. The monitoring sensor 42 generates a synchronizing signal to indicate the drawing-start timing of each scan in response to each detection of the monitor beam.
The laser beam reflected by the curved surface mirror 15 is converged in the main scanning direction, but is diverged in the auxiliary scanning direction, so it is re-converged by the monitoring cylindrical lens 41 in the auxiliary scanning direction to form a spot on the monitoring sensor 42.
In FIG. 4, a y-direction and a z-direction are defined in a plane P that is perpendicular to the monitor beam incident on the monitoring sensor 42. The y-direction is a scanning direction of the monitor beam on the plane P and the z-direction is perpendicular to the y-direction on the plane P. When the main meridian of the monitoring cylindrical lens 41 is projected onto the plane P, the direction of the main meridian N is parallel to the z-direction. The main meridian is perpendicular to a generatrix of the monitoring cylindrical lens 41 and indicates a direction of main refractive power thereof.
The laser beam is incident on the polygonal mirror 14 with an inclination in the auxiliary scanning direction, which causes a skew distortion in the deflected laser beam when a scanning angle W is not zero. The skew distortion increases as the absolute value of the scanning angle w increases. It should be noted that the scanning angle w is defined as an angle formed between the center axis of the laser beam deflected by the polygonal mirror 14 and a reference light ray that points at the center of the scanning range on the photoconductive drum 18. That is, when the deflected laser beam points at the center of the scanning range, the scanning angle W is zero. The scanning angle W has a positive value at the side of the monitoring flat mirror 40 with respect to the reference light ray and has a negative value at the other side. The deflected beam of the scanning angle W=47.0.degree. is incident on the monitoring mirror 40.
FIGS. 5A, 5B, 5C and 5D show the skews of the deflected laser beams at the scanning angle W=0.0.degree., W=20.0.degree., W=43.0.degree. and W=47.0.degree., respectively. In each of the figures, a horizontal axis means the main scanning direction and a vertical axis means the auxiliary scanning direction.
Since the drawing anamorphic lens 16 acts as a correcting optical system for correcting effect of the skew of the deflected laser beam, the skew distortion has little effect on the beam spot formed on the photoconductive drum 18.
However, the monitor beam travels to the monitoring sensor 42 through the monitoring cylindrical lens 41 without passing the drawing anamorphic lens 16, which disturbs the wavefront of the monitor beam on the monitoring sensor 42 as shown in FIG. 6. The disturbance of the wavefront distorts the shape of the monitor beam spot on the monitoring sensor 42. FIGS. 7A, 7B and 7C are spot diagrams to show the shapes of the monitor beam spots at the point in front of the sensor by 2 mm (the side of the monitoring cylindrical leas 41), on the sensor and at the point in the rear of the sensor by 2 mm, respectively. The monitor beam is not sufficiently converged on the monitoring sensor 42 as shown in FIG. 7B, which blunts the rising edge of the signal generated by the monitoring sensor 42. Therefore, the scanning-start points determined by the signal from the monitoring sensor 42 may vary widely, which reduces printing quality.